1. Field of the Invention
The present invention generally relates to a copying apparatus and, more specifically, to a copying apparatus having a function of forming color copies as well as a function of forming outline images.
2. Description of the Prior Art
Recently, devices capable of forming not only black images but color images such as red, blue and so on have been developed in the field of image forming apparatuses such as copying machines employing electrophotography process. More specifically, a plurality of developing devices are arranged around a photoreceptor, and a color developing device containing developer with color toner can be selectively mounted and used by replacing other color developing devices.
In consideration of the recognition that an outline of an image is very important, comprising much information and representing characteristics of the image, the applicant of the present application has proposed a method for forming an outline image, in which a second charging apparatus is provided between an image exposing apparatus and a developing device, and only outlines of a picture are extracted to form outline images, and line images are directly reproduced as the line images by the operation of the second charging apparatus (disclosed in U.S. Ser. No. 16,716 and U.S. Ser. No. 16,717).
Let us consider a case in which the function of forming outline images is implemented in a unit (hereinafter referred to as an outline unit) compatible with the developing devices, and the unit is mounted on a copying machine capable of mounting a plurality of developing devices in place of one of the developing devices. In order to turn a latent electrostatic image of the outline portion to a toner image by a developing device, the outline unit must be arranged on the upstream side of at least one developing device in association with the direction of movement of a photoreceptor so as to enable selection of developing devices arranged on the downstream side of the outline unit, in order to prevent miscopying.
If there are a plurality of developing devices which can be selected, the copying apparatus should preferably be controlled to prevent operational error as much as possible, without prohibiting image formation even in the case the image formation is started with the developing device not selected.